Valve devices for electro-pneumatic brake control devices for controlling vehicle parking brakes are known, for example, from DE 103 36 611 A1 or EP 1 571 061 A1. These known brake control devices are used in brake systems provided not only with a service brake, which can be actuated by means of a brake pedal, but also with a parking brake (often also referred to as a handbrake), which can be actuated by means of an electrical signal transducer.
In these known brake systems, the parking brake is regularly applied by means of brake cylinders designed as spring-actuated brake cylinders. In order to release the parking brake, the spring-actuator part of the spring-actuated brake cylinders is pressurized with compressed air. For this purpose, air is admitted to the spring-actuator part. The necessary pressure for air admission is supplied from a compressed-air reservoir. However, this pressure supply is not permanently turned on, but may even be shut off. Furthermore, the pressure in the spring-actuated brake cylinder may also be lowered, and so the spring-actuator part may be vented.
To control the pressure in the spring-actuated brake cylinder, conventionally, a relay valve is provided by means of which the pressure supply from the compressed-air reservoir tank to the spring-actuator part of the spring-actuated brake cylinders can be controlled. Control is exercised with the assistance of electro-pneumatic valve devices, especially, by means of electrically actuatable solenoid valves, that regulate a control pressure supplied to the relay valve.
In the known brake system described in DE 103 36 611 A1, a bistable valve, or, in other words, a valve that can occupy two stable conditions and that, in the event of a power failure, maintains the condition set at that instant, is used for this purpose. Furthermore, a holding valve is connected between the bistable valve and the control input of the relay valve in this brake system. By means of the bistable valve and of the holding valve, the pressure at the control input can be held, raised or lowered. The pressure at the output of the relay valve varies correspondingly. In this way, the parking brake can be released or applied by means of electrical signals to the bistable valve and holding valve.
Because of its construction, however, the bistable valve is complex and therefore can be expensive. Furthermore, as noted above, the bistable valve remains in its previous condition in the event of failure of the electrical power supply. Thus, a vehicle having such a brake system, in the event of failure of the electrical power supply, cannot be parked such that the spring-actuator part of the spring-actuated brake cylinder is vented and, as a result, the parking brake is applied.
Electromagnetically actuatable multi-way valves of less complex construction have been proposed in DE 35 01 708 A1. In particular, it was proposed that two valves disposed opposite one another be provided in a multi-way valve. Armatures that cooperate with a coil disposed between the two valves are associated with each of these two valves. The springs associated with these armatures are designed such that different magnetic forces are necessary for actuation of the two valves. These magnetic forces are generated by application of a current flowing through the coil. Because a separate armature is associated with each valve, mutually independent actuation of the two valves is possible by appropriate energization of the coil.
However, in the de-energized state of this known valve, the input of the valve unit is in communication with the output leading to the consuming load. If such a valve unit were to be used instead of the bistable valve and holding valve discussed above, the full reservoir pressure might be applied to the control input of the relay valve in the event of failure of the electrical power supply, thus, potentially causing the parking brake to be released by the resulting admission of air to the spring-actuator part of the spring-actuated brake cylinders. These known valves might not, therefore, be best suited for controlling the pressure in a spring-actuated brake cylinder of a parking brake.